Unit testing (check) warnings when building with Travis

Check unit testing complains is expecting casts, e.g.

tests/interval.c: In function ‘interval_creation’:
tests/interval.c:51:3: warning: passing argument 1 of ‘_ck_assert_msg’ makes integer from pointer without a cast [enabled by default]
   ck_assert ( i );

testing locally does not give this kind of warnings.